Ionic Half Equations (Cambridge (CIE) IGCSE Chemistry) : Revision Note

Use our revision notes to learn how to write and balance ionic half equations for IGCSE chemistry. Learn more.

Alexandra Brennan

Written by: Alexandra Brennan

Reviewed by: Stewart Hird

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Ionic half equations

Extended tier only

  • Electrochemistry is concerned with the transfer of electrons

    • This is why the definitions of oxidation and reduction are in terms of losing or gaining electrons, not oxygen 

    • Oxidation is the loss of electrons 

    • Reduction is the gain of electrons

  • As the ions come into contact with the electrode:

    • Electrons are lost or gained

    • The ions form neutral substances

    • These substances are discharged as products at the electrodes

  • At the anode, negatively charged ions lose electrons 

    • So, oxidation occurs at the anode

  • At the cathode, the positively charged ions gain electrons 

    • So, reduction occurs at the cathode

  • Ionic half-equations only show half of what is happening in a reaction involving electron transfer

    • The ionic half-equation for oxidation shows the loss of electrons

    • The ionic half-equation for reduction shows the gain of electrons

  • Ionic half-equations must have the atoms and charges balanced 

Writing ionic half-equations

Metals

  • Metals are positive ions, e.g. Li+, Cu2+, Al3+ 

  • If a metal is produced during electrolysis:

    • The metal ions will gain electrons to form metal atoms

    • The metal ions will be reduced

  •  The charge on the metal ion indicates the number of electrons that will be gained

    • For example:

Li+ + e → Li

Cu2+ + 2e → Cu

Al3+ + 3e → Al

Non-metals

  • Non-metals are typically negative ions, e.g. Cl, Br, OH 

    • The hydrogen ion, H+, is an exception to this

Hydrogen ion / hydrogen half-equation:

  • If hydrogen is formed during electrolysis:

    • Two hydrogen ions will gain 2 electrons to form hydrogen, H2  

    • Hydrogen ions are reduced

2H+ + 2e → H2 

Halide ion / halogen half-equation:

  • If a halogen is formed during electrolysis:

    • Two halide ions will lose 2 electrons to form the halogen

    • Halide ions are oxidised

2X → X2 + 2e 

Hydroxide ion / oxygen half-equation:

  • The ionic half-equation for hydroixde ions forming oxygen is more challenging to balance

    • Hydroxide ions lose electrons to form oxygen, O2, and water, H2O

    • Hydroxide ions are oxidised

4OH → O2 + 2H2O + 4e 

Table of reduction and oxidation reactions at the electrodes

Electrolysis of

Anode reaction

Cathode reaction

Molten lead(II) bromide, PbBr2 

2Br → Br2 + 2e

Pb2+ + 2e → Pb

Concentrated aqueous sodium chloride, NaCl

2Cl → Cl2 + 2e

2H+ + 2e → H2 

Dilute sulfuric acid, H2SO4 

4OH → O2 + 2H2O + 4e 

2H+ + 2e → H2 

Examiner Tips and Tricks

  • To help you remember the definitions of oxidation and reduction, use OIL RIG

    • Oxidation Is Loss (of electrons) Reduction Is Gain (of electrons)

  • To help you remember where oxidation and reduction take place, use a RED CAT and AN OX 

    • REDuction at the CAThode

    • ANode for OXidation

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Alexandra Brennan

Author: Alexandra Brennan

Expertise: Chemistry Content Creator

Alex studied Biochemistry at Newcastle University before embarking upon a career in teaching. With nearly 10 years of teaching experience, Alex has had several roles including Chemistry/Science Teacher, Head of Science and Examiner for AQA and Edexcel. Alex’s passion for creating engaging content that enables students to succeed in exams drove her to pursue a career outside of the classroom at SME.

Stewart Hird

Reviewer: Stewart Hird

Expertise: Chemistry Lead

Stewart has been an enthusiastic GCSE, IGCSE, A Level and IB teacher for more than 30 years in the UK as well as overseas, and has also been an examiner for IB and A Level. As a long-standing Head of Science, Stewart brings a wealth of experience to creating Topic Questions and revision materials for Save My Exams. Stewart specialises in Chemistry, but has also taught Physics and Environmental Systems and Societies.

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